9

FN6050.7

July 31, 2006

250kbps versions can operate at full data rates with lengths

in excess of 1000’.

Twisted pair is the cable of choice for RS-485/RS-422

networks. Twisted pair cables tend to pick up noise and

other electromagnetically induced voltages as common

mode signals, which are effectively rejected by the

differential receivers in these ICs.

To minimize reflections, proper termination is imperative

when using the 5Mbps device. Short networks using the

250kbps versions need not be terminated, but, terminations

are recommended unless power dissipation is an overriding

concern.

In point-to-point, or point-to-multipoint (single driver on bus)

networks, the main cable should be terminated in its

characteristic impedance (typically 120

Ω) at the end farthest

from the driver. In multi-receiver applications, stubs

connecting receivers to the main cable should be kept as

short as possible. Multipoint (multi-driver) systems require

that the main cable be terminated in its characteristic

impedance at both ends. Stubs connecting a transceiver to

the main cable should be kept as short as possible.

Built-In Driver Overload Protection

As stated previously, the RS-485 spec requires that drivers

survive worst case bus contentions undamaged. These

devices meet this requirement via driver output short circuit

current limits, and on-chip thermal shutdown circuitry.

The driver output stages incorporate short circuit current

limiting circuitry which ensures that the output current never

exceeds the RS-485 spec, even at the common mode

voltage range extremes. Additionally, these devices utilize a

foldback circuit which reduces the short circuit current, and

thus the power dissipation, whenever the contending voltage

exceeds either supply.

In the event of a major short circuit condition, these devices

also include a thermal shutdown feature that disables the

drivers whenever the die temperature becomes excessive.

This eliminates the power dissipation, allowing the die to

cool. The drivers automatically re-enable after the die

temperature drops about 15 degrees. If the contention

persists, the thermal shutdown/re-enable cycle repeats until

the fault is cleared. Receivers stay operational during

thermal shutdown.

Low Power Shutdown Mode (Excluding ISL81487)

These CMOS transceivers all use a fraction of the power

required by their bipolar counterparts, but the ISL8487 and

ISL81483 include a shutdown feature that reduces the

shutdown whenever the receiver and driver are

period of at least 600ns. Disabling both the driver and the

receiver for less than 50ns guarantees that shutdown is not

entered.

Note that receiver and driver enable times increase when

enabling from shutdown. Refer to Notes 5-9, at the end of

the Electrical Specification table, for more information.

FIGURE 6. DRIVER OUTPUT CURRENT vs DIFFERENTIAL

OUTPUT VOLTAGE

FIGURE 7. DRIVER DIFFERENTIAL OUTPUT VOLTAGE vs

TEMPERATURE

DIFFERENTIAL OUTPUT VOLTAGE (V)

0123

45

0

10

20

30

40

50

60

70

80

90

-40

0

50

85

2

2.2

2.4

2.6

2.8

3

3.2

3.4

3.6

TEMPERATURE (°C)

-25

25

75

ISL8487, ISL81483, ISL81487